The Mohr titration has been found to work well with the dilute and, worst case, faintly-coloured wall-wash samples. Interferences are very rare.
The method employed is as per Vogel’s Quantitative Inorganic Analysis, Section III,23,A, adapted to low umolar amounts of chloride per aliquot.
The certainty of end-point detection is improved by adding 5 umoles of KCl at the first end-point, and titrating back over it to obtain a second reading. Proceed as follows –
- 25ml aliquot of sample
- add 75ml DIW
- add 2ml of potassium chromate indicator
- titrate against silver nitrate (0.01M) to end-point
- add 5ml of standard 1mM KCl
- titrate again to the end-point
gmoles of chloride in aliquot = gmoles of silver titrated
Silver nitrate solution 0.01M:
- weight-out close to 0.849g of AR silver nitrate
- make up to 500ml with DIW
- store in dark-glass bottle
- standardise against sodium chloride 0.01M
Potassium Chromate Indicator:
- dissolve 5g of AR potassium chromate in 100ml of DIW
- Blank readings are significant and contribute an error of something like +7 umole. The volume of chromate indicator, and the volume of the solution at end-point both contribute to the blank value.
- Diluting the sample out to 100ml makes for more consistent blank readings and diminishes the likelihood of interferences.